Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1999-10-28
2002-04-23
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S044000, C423S059000, C423S149000, C423S215500, C423S481000, C423S522000, C423SDIG001, C110S216000, C110S219000, C110S345000, C110S346000, C588S253000, C588S253000, C588S253000
Reexamination Certificate
active
06375908
ABSTRACT:
The invention relates to a process for recovering raw materials from wastes and residues, in particular for recovering heavy metals, in which a liquid or paste-like feed mixture and/or a feed mixture comprising comminuted or milled constituents is first produced and the recovery of the raw materials from the feed mixture is carried out by means of a chemical-thermal treatment and the waste gases are conveyed from the furnace through a multistage filter unit, where the first filter in each case is configured as a hot filter and the waste gases are subsequently cooled and flow through at least one second filter, are preheated and are then burnt at a high temperature. The invention further relates to an apparatus for carrying out the process.
Production processes, particularly in the production of a wide variety of products, result in formation of wastes or production-related residues, usually in the form of mixtures containing heavy metals such as chromium, zinc, nickel, copper, lead, etc., often in combination with organic substances, with the proportion of chromium generally predominating.
The separation of these mixtures to recover valuable raw materials is desirable, but is very difficult because of the wide differences in the chemical properties of these mixtures. For example, chromium(VI) oxide or chromium(III) oxide (Cr
2
O
3
) or zinc can be present in the liquid, paste-like or solid comminuted or milled wastes or residues.
Since the recovery of the heavy metals is very complicated and comparatively uneconomical, wastes containing heavy metals are, for example in a physicochemical plant, treated in such a way that the harmful materials are reduced, i.e. made inert, in other words made difficult to leach. The filter cake obtained in this way can then be stored permanently in a repository for hazardous waste.
For example, a process for recovering chromium known from Bayer AG is operated in a high-concentration region and only certain solutions which have been formed in Bayer's own processes, which contain particular contaminants and which have been used as additives for processing chromium ores are used. Here, the heavy metals are brought from the liquid to the solid phase, i.e. the first step is a chemical treatment in which the highly concentrated chromium-containing wastes are precipitated to form sludges. After these sludges have been dried, the wastes can then be processed further in a metallurgical plant.
DR-A-3514471 discloses a process and an apparatus for separating arsenic from the hot waste gas formed in the metallurgical processing of arsenic-containing materials in melt processes. In this process, the waste gases, which contain arsenic-containing impurities or condensates and dusts containing valuable metals, are purified in a multistage filter unit. To achieve deposition of arsenic-free dusts of valuable metals, the waste gas is cooled immediately after the melt process and treated by addition of a reducing agent so that unstable arsenic-containing compounds are formed. As a result, the arsenic is kept in the gas phase in stable form and cannot deposit on the valuable-metal-containing dusts which are separated out in the hot gas filter.
The arsenic condensates are then separated off in a second, downstream cold filter.
However, such a process does not make it possible to recover heavy metals from waste and residues.
Furthermore, WO-A-91/05681 describes a process for recovering chromium from wastes by formation of water-soluble chromates. For this purpose, dried chromium-containing material is comminuted and heat-treated with addition of alkaline reactants and of oxidants in a closed chamber to which oxygen is supplied. The water-soluble chromates formed are subsequently rinsed out. However, this process is not suitable for producing dusts or flakes containing noble metals.
Finally, U.S. Pat. No. 3,968,756 describes a process for the incineration of chromium-containing sludge.
Otherwise, highly concentrated wastes are, in accordance with current strict environmental regulations, packed in containers or the like and preferably stored permanently in underground repositories.
In the case of slightly contaminated wastes which have a certain minimum calorific value, it is possible for them to be burnt in a waste incineration plant. However, the residues formed in the incineration, for example fly ash and slag, then have to be stored in repositories for hazardous waste because of the increased heavy metal content. In addition, the waste incineration plant has to be connected to a downstream afterburning plant in order to remove the pollutants, e.g. dioxin, formed in the first combustion of organic substances from the waste air. For this purpose, the afterburning plant has to be operated at a considerably higher temperature than the actual waste incineration plant. The disadvantage of this is that, as a result of the high temperature, the chromium(III) oxide produced in the first thermal treatment is converted back into readily soluble chromium(VI) oxide and can then even leave the plant in an uncontrolled fashion. A similar effect occurs in the case of zinc.
In exceptional cases, small amounts of the residues having a restricted composition can be used as additives in metallurgical plants after appropriate pretreatment.
These examples show that only an extremely small proportion of the valuable heavy metals is reused. The permanent storage of the major part of the heavy metals from residues and wastes in repositories for hazardous waste is the economically and ecologically least favourable variant.
It is an object of the invention to develop a process for recovering and separating raw materials from wastes and residues, which can be implemented using simple means operates very reliably and as far as possible forms no further wastes or residues and by means of which the residues and wastes which have already been stored in repositories can be reprocessed.
According to the invention, the separation of the raw materials is carried out in a process of the type mentioned at the outset by means of a chemical-thermal treatment in a furnace through which air flows, by first mixing, as a function of the composition, the liquid, paste-like and/or solid feed mixture with additives blowing it together with an oxidizing or reducing agent into the furnace. Subsequently, the feed mixture is subjected to the chemical-thermal treatment in the furnace by first mixing, as a function of their composition, the liquid, paste-like and/or solid feed mixture with additives, blowing the feed mixture together with an oxidizing or reducing agent into the furnace in which the furnace atmosphere flows at a predetermined flow velocity through the feed mixture at a predetermined temperature so as to form, as a function of the air composition and temperature, low-density flakes containing heavy metals which are conveyed by means of the air flow velocity established in the furnace through the filter unit.
Suitable additives are aluminium-, iron-, chlorine- or sulphur-containing materials, and also milled plastics or granulated plastics as reducing agents. For use in chromium-containing chamotte bricks, possible additives are aluminium oxide in a ceramic mix or iron oxide in the case of specific alloys.
The oxygen content, the rate of the chemical reaction and the density of the flakes formed are the most relevant process parameters and determine the necessary flow velocity in the furnace in the individual case. In addition, the flow velocity is dependent on the heavy metal to be recovered and also on whether a rotary tube furnace or a fluidized-bed furnace is used.
The thermal treatment is carried out in a reducing/oxidizing atmosphere at a temperature in the range from 350° C. to above 700° C. depending on the respective feed mixture. The furnace temperatures required in the individual case depend on whether heavy metals are to be recovered, on the composition of the feed mixture and on the end products which are to be obtained (mineralization temperature). Thus, in the case of chromium as ch
Kaszas Tiberiu
Kaszas-Savos Melania
Baker & Botts LLP
Griffin Steven P.
Vanoy Timothy C.
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